1. Field of the Invention
The present invention relates to a boat propulsion unit, in particular it relates to an electric boat propulsion unit.
2. Description of the Related Art
In recent years, a boat propulsion unit for small boat applications in which an operation mode of an electric motor includes a forward operation mode, a reverse operation mode, and a stop mode, as well as the output of the electric motor is controlled by rotating a throttle grip has been disclosed. The position designated by the throttle grip is detected by using a potentiometer. Then, the controller regulates the operation mode and the output of the electric motor based on the signals received from the potentiometer.
Such a boat propulsion unit is configured to prohibit driving of the electric motor when some abnormality, such as a wire disconnection and short-circuiting has occurred on the potentiometer, regardless of the position designated by the throttle grip. This configuration is used for the sake of safety, but the consequences of its operation are very disadvantageous because the boat can no longer be propelled (controlled) by using the boat propulsion unit.
JP-A-Hei 6-249039 and JP-B-3525478 disclose a configuration in automobiles that operates such that, when an acceleration sensor cannot detect a depression amount of an accelerator pedal, engine output power is controlled based on the output of an accelerator switch which is activated and deactivated in accordance with the depression amount of the accelerator pedal.
However, the accelerator pedal according to JP-A-Hei and JP-B-3525478 is only used for designating an engine output power. JP-A-Hei 6-249039 and JP-B-3525478 do not clarify how to control an operating mode of a vehicle when the position designated by a designation device cannot be detected in an arrangement where the operating mode and the output power is designated by a rotatable designation device.